I had noticed a slight rhythmic rattling noise when I used my SuperMax 19-38, and was concerned that it might indicate a part wearing out, or other serious problem. My conveyor belt appeared to have stretched over the past five years, and I wondered if it needed replacing. The noise disappeared if I loosened the belt tension, but that resulted in the belt slipping, leaving ridges in the wood.
The source of the noise appears to have been dirt--black, slightly sticky residue that coated the rollers and inside of the conveyor belt. At the advice of a tech support person at Laguna (where SuperMax now resides), I removed the conveyor assembly and belt, and cleaned them as best I could. Once reassembled and tensioned, everything seems to be working properly.
Since I'm always wearing my hearing protection whenever I turn on the dust collector and sander, I have no idea how long ago the noise started. I don't even recall how I happened to be operating the conveyor by itself, but I'm very glad that I did!